Modern offset printing utilizes Kirschner printing, i.e., four-color printing in which a color picture is divided into four colors: cyan (C), magenta (M), yellow (Y) and black (B). For the ink, its color and consistency are invariant, and thus the outputs of the printing are 0 and 1, which, respectively, represent whether the ink exists or not without any middle grayscale. This thus determines that the image to be printed should consist of binary image elements. A process for transferring a grayscale image into a two-value image is called a screening, in which a continuous-tone image is decomposed into screen dots. In the screened image, the actually gradation of the image is embodied by the size and density of screen dots. In the iconography, a process for screening an image is called digital halftone process. If distances between screen dots are constant and sizes thereof are variant, then the process is called amplitude modulation screening; however, if the distances are variant and the sizes are constant, then the process is called frequency modulation screening.
When a printed image is viewed by an amplifying device, it is found that, the microscopic dot-particles with the same color, which can be also seen by the naked eye, are arranged regularly. In some images, the dot-particles are arranged regularly in the center while increase in size as the local grayscale increases, wherein such images are generated using an amplitude modulation screening process and the dot-particles therein with various shapes are called screen dots in the art. In other images, the dot-particles are smaller and have same sizes, and the dot-particles are dense at high grayscale but sparse at low grayscale. Such images are generated by a frequency modulation screening process and the dot-particles therein are also called screen dots that are different from those in the amplitude modulation screen.
The amplitude modulation screen has a screen shape in which the screen dots vary in shape and size as grayscale increases. In addition to purely circular screen dots, the additional screen shape further has linear screen dots, square-circular screen dots, square screen dots, oval screen dots, and diamond screen dots and so on. These screen dots belong to traditional amplitude modulation screen dots.
In the traditional printing, amplitude modulation screen dots are frequently used. In this case, since there is a common phenomenon of screen dots enlargement in the plate printing, the greater screen dots enlarge, the greater the output gradation of printing losses. Since the screen dots enlargements are different in degree, based on geometrical deduction, purely circular screen dots have the smallest perimeter among others, the purely circular screen dots thus will be enlarged less than other regular screen dots. However, if the circular screen dots connect with each other, the coefficient of enlargement will become much higher, which will cause screen dot ink to be accumulated in and around shade-tone area, such that the shade-tone section has no gradation.
The inventor found that, other screen dots in additional to the purely circular screen dots have two typical characteristics belonging to the amplitude modulation screen as follow:                1) individual screen dots have relative larger perimeters compared to the purely circular screen dots;        2) the screen dots are only arranged in two orthogonal angles, θ and 90+θ, respectively.        
Meanwhile, the orthogonal angles mentioned in 2) are also common to the traditional screen dots. The inventor found that, based on many practical printing experiences in combination with the geometrical deduction, it can be deduced that the orthogonal arrangement of screen dots trend to cause the CMYK four-color printing to hit a net or produce a problem of texture under the limited angle selection and inappropriate control, which finally affects the output quality of printing.